Typically, production strings in wells have an SSV which is controlled from the surface. The SSV is typically a spring-loaded flapper which is pushed into the open position by downward movement of an open tube called the "flow tube." The flow tube is actuated by an actuating piston which is, in turn, a part of a control circuit for selective opening and closing of the SSV from the surface of the well. Many different designs have been used in the past to control the opening and closing of the SSV. Typically, a control line is run from the surface to the actuating piston and a return spring acts on the actuating piston in a direction opposite the hydrostatic force put on the piston by the column of fluid in the control line to the surface. The piston is typically an annular shape or it can have a cylindrical or rod shape. The spring is made sufficiently stiff so as to withstand the anticipated hydrostatic force for the depth to which the valve is to be installed. Yet other designs have included pressurized gas chambers which act on the backside of the actuating piston to resist the hydrostatic pressures anticipated in the control line. The pressurized gas chambers contain oil so that the actuating piston seals are lubricated.
Annularly shaped pistons have been waning in popularity due to the numerous seals required, all of which increase the prospects for leakage and malfunctioning of the valve. Another main concern of any design for a control system for an SSV is the failure mode if certain seals malfunction. It is important to have failsafe operation of the SSV and, thus, the fewer situations that can arise where the valve fails open, the more desirable is the control system design and the valve which goes with it.
Some designs in the past have used pressure-balancing between the top side and bottom side of the actuating piston, coupled with fairly complex shuttle valving to allow for normal operation of the valve between an open and closed position. While use of the concept of pressure-balancing has enabled a significant reduction in the size of the return spring, other complications introduced into the system to make such a design operable have created a new set of operational issues, detracting from the desirability of the equalizing-type designs which use a complex shuttle valve. What is yet to be developed and what is an object of this invention is to provide a simple design which has minimal possibilities for fail-open operation and which is simple to build and install and reliable to operate.
Some of the patents which illustrate the prior designs discussed above are U.S. Pat. Nos. 5,564,501 and 4,676,307. Also of general interest in the area of SSV control systems are U.S. Pat. Nos. 4,252,197 and 4,448,254.
Accordingly, one of the objects of the present invention is to provide a control system where the actuating piston, which is a rod type, is in pressure balance. In combination with this objective, which is accomplished by the provision of a balance line to the surface, the actuating piston is configured in such a way so as to meet the objective of the invention of minimizing, and in certain situations eliminating, fail-open modes of the valve. These and other objectives will become apparent to one skilled in the art from a review of the preferred embodiment described below.